JDK-8319801 has introduced new C2 nodes cmpFastLockLightweight and cmpFastUnlockLightweight which use the box register as temp register, but the effects don't specify this behavior as a USE_KILL effect.
It seems like the overwritten register content is never used, so, we currently don't see errors.

There are several solutions:
- Use a fixed register and USE_KILL effect.
- Add a separate temp register.
- Try to use one register less.
- Ignore the bug and check that nothing can go wrong.

In the long term, we can hopefully remove the box when the legacy locking mode is removed.